bacterial and fungal host specificity plant pathology 602 ...plant pathology 602 plant-microbe...

Plant Pathology 602Plant-Microbe Interactions

Sophien [email protected] Ohio State UniversityOhio Agricultural Research & Development Center

LectureGene for gene interactions

Gene for gene interactions (SK) Plant disease resistance genes (R genes) (SK) Genetic engineering/breeding for resistance (SK)

Fungal/oomycete effector proteins (SK) What types of resistance can R genes mediate? (SK) Discussion

Bacterial Type III secretion Avr proteins (DC) Bacterial pathogenicity islands (DC)

Bacterial and fungal host specificityLectures 9-11

The gene for gene model Plant disease resistance genes (R genes)


Outline - gene for gene interactions

The gene for gene model

Plant disease resistance genes (R genes)



Plants are resistant to most pathogens

Species A Species B

Species C Species D

Plants are resistant to some strains ofa pathogen but not to others

Plant cultivars or lines

Pathogenstrains orraces

Gene for gene model

Plant breeders recognized that resistance to diseaseoften segregates as a single dominant or semi-dominant gene (locus). These became known as Rgenes.

In the 1940s, H.H. Flor performed genetic analyseson both plant and pathogen and defined a modelthat provides a genetic basis for the variation

Gene for gene model

Flor studied the interaction between flax and thefungal pathogen flax rust (Melampsora lini)

He found that whereas flax genes for resistancewere dominant, rust genes for virulence wererecessive

This ultimately led to the current view that pathogenscontain a variety of molecules, encoded by dominantavirulence (Avr) genes, that trigger defenseresponses in plants carrying the corresponding Rgene

Gene for gene model- quadratic checkPathogen race A

(AvrD/AvrD or AvrD/avrD)Pathogen race B

(avrD/avrD)

Plant cultivar A (Rd/Rd or Rd/rd)

Plant cultivar B (rd/rd)

Gene for gene model- haploid pathogenPathogen race A

(AvrD)Pathogen race B

(no AvrD or inactive AvrD)



One biochemical expression of the gene for genemodel is direct recognition

R genes products are receptors for ligands encodedby Avr genes

Ligand (Avr) binding to R receptor triggers asignaling cascade leading to inactivation of defensemechanisms including hypersensitive response (HR)

Gene for gene- direct recognition (elicitor-receptor model)

Gene for gene- direct recognitionPathogen race A


(avrD/avrD)



Resistance Disease

DiseaseDisease

Recent data suggest that in many cases no directphysical interactions between R and Avr proteinsoccur

This led to the alternative indirect perception model,one example of which is the guard model

R proteins do not directly detect Avr proteins butrather detect the biochemical perturbations the Avrproteins cause in plant cells

Gene for gene- indirect perception (guardmodel) The discovery that one’s kitchen has been invaded by mice

is often made indirectly. The holes chewed in the mueslibag and the teeth marks on the corn flakes box are adead give away. Although you have not seen the mouse,you deploy your defensive weapons, and if successful,succeed in protecting your valuable goods from theinvasion, hopefully before all of your food has beeneaten. Recent research results indicate that plants alsomake use of such indirect surveillance systems to protectthemselves from being consumed by pathogens. Ratherthan wait for a direct observation of the pest, plantsappear to activate their defenses as soon as pathogen-induced damage is detected.

Indirect perception model- Roger Innes Plant Physiol 135:695

Indirect recognition model

Susceptible Plant

Pathogen

Suppress defense,enhance susceptibility,

disease symptoms

Resistant Plant

Pathogen

HR

Examples: AvrRpt2 - RIN4 - Rps2

Examples: Avr2 - Rcr3 - Cf-2

Goes beyond one Avr gene-one R gene model

Explains why R proteins can recognize unrelatedpathogens (example tomato Mi gene recognizesnematodes and aphids)

Virulence and Avirulence genes are functionallysimilar. The term avirulence is misleading. Thus, newterminology: virulence or avirulence=effector genes

What are the effector targets and what is their functionin defense?

Implications of indirect perception model

Gene for gene- other genes are important too!Pathogen race A


(avrD/avrD)



Genes for genes interactionsPathogen races (AvrS and AvrD)

Plant cultivars(Rs and Rd)

The interaction between: Pathogen Avirulence (=virulence=effector) genes

encoding Avr products Plant Effector (=virulence) Targets (ET) that are the

cellular targets of pathogen effectors Plant Resistance genes encoding Resistance gene

products (R proteins) that detect theEffector/Effector Target complexes or thebiochemical perturbations caused by the effectors

Gene for gene interactions

Hypersensitive response

R gene mediated resistance and the HR

R gene mediated resistance is often (but not always)associated with a localized cell death response, thehypersensitive response (HR)

The HR is also accompanied by a complex set ofbiochemical and physiological changes (defenseresponse)

In some cases, HR cell death is not observed-example: Extreme resistance to viruses such as withRx potato resistance to PVX

The HR and other defense responses are generallynot specific to a particular pathogen

The key to the gene for gene model is that specificityof resistance lies in the recognition of a particular Avrproduct by a particular R gene product and not in thesubsequent defense response

Gene for gene- recognition vs. response

Can an Avr gene from a particular pathogen confer

avirulence in a gene for gene manner to an unrelatedpathogen?

Fungal Avr gene: Cladosporium fulvum Avr9 transfer

to Potato Virus X


Kamoun et al. 1999 MPMI 12:459

166K 25K 8K CP12K

PVX

166K 25K 8K CP12K

PVX-AvrAvr9

Potato Virus X (PVX)

Potato Virus X (PVX)

Virus is applied byrubbing a lower leaf

PVX virions

Virus spreads systemically andcauses mosaic symptoms

Gene for gene model- specificity is the key!Potato Virus X Potato Virus X (Avr9)

Tobacco

Tobacco (Cf-9)

Gene for gene model- specificity is the key!

Potato Virus X Potato Virus X (Avr9)

Fungal gene Avr9 gene confers specific avirulence toPotato Virus X only on tobacco plants that containthe corresponding Cf-9 R gene

Similar results obtained with bacterial AvrPto,oomycete inf1 avirulence genes, etc…

Gene for gene interactions can be transferred toother organisms


HR and other related defense responsestriggered by specific Avr elicitors are equallyeffective against viruses, bacteria, fungi,oomycetes, nematodes and insects


The gene for gene model

Plant disease resistance genes (R genes)



Pathogen recognition

Activation of signaling cascade leading to HR

and defense response

Rapid evolution of new recognition specificities

Plant disease resistance genesProperties of R genes and their products

Dozens of R genes have now been cloned from adiversity of plants

These R genes recognize viruses, bacteria, fungi,oomycetes, nematodes and insects

Transposon-based gene tagging used to clone Rgenes from maize, tobacco, tomato, and flax

Map-based positional cloning used for tomato,barley, rice and Arabidopsis

Plant disease resistance genesCloning

Several classes of R genes are known

R genes from diverse plants share similar

structural features independently of thepathogens they target

R genes often occur in complex gene clusters

Plant disease resistance genesMolecular characterization

Plant disease resistance genesMajor protein motifs found in various classes of

products of R genes (R proteins)

Coil-coil motifs (CC) Toll-Interleukin Domain (TIR) Nucleotide Binding Sites (NB) Leucine Rich Repeats (LRR) Kinase Nuclear localization signal (NLS) DNA binding (WRKY)

Plant Cell

Leucine-Rich RepeatsPlant disease resistance genes

Major protein motifs found in various classes of products of Rgenes (R proteins) are also found in animal defenseresponse proteins

Plant disease resistance genes

Resistance proteins are receptor-

like proteins that localize in

different cellular compartments:

apoplast (surface of the plant)

cytoplasm

nucleus

Plant Cell

Plant disease resistance genesChromosomal organisation of an R gene cluster

NBS LRR

Hypervariable Region

Plant disease resistance genesChromosomal organisation of an R gene cluster in

resistant and susceptible lines: The Cf-4/9 cluster

Introgressed into tomato from the wild speciesrelatives: Lycopersicon pimpinellifolium (Cf-9) and L.hirsutum (Cf-4)

Complex clusters of Hcr9 genes (homologs of Cf-9) Located in chromosome 1 At least five functional Hcr9 genes have been

identified encoding at least four distinct recognitionspecificities

Plant disease resistance genesChromosomal organisation of an R gene cluster in the


Parniske andJones 1999PNAS 96:5850

Plant disease resistance genesChromosomal organisation of an R gene cluster in the


Complex patterns of separation and rare

recombination between members of the clusters andbetween members of different clusters allow high

mutation rates and occasional reshuffling of genes

This probably contributes to the emergence of newspecificities

Plant disease resistance genesSummary

Recognition specificity probably lies in LRR domainin a typical R protein

Signal transduction is probably initiated by CC/NBSregion and/or kinase domain

R protein associate with co-receptors (resistasome)

High mutation rates and gene rearrangements incomplex clusters probably allow rapid evolution

Exploiting R genes to engineer resistantplants

Cloning of R genes allow direct transfer of genes tocultivars of interest and from one crop to another

Great diversity of R genes in plants

Challenge is to identify novel R genes and theircorresponding targets

Heterologous transfer of R genes works best withinthe same botanical family

Strategies based on “artificial” or “engineered” HR

Resistance Gene Analogs (RGAs)-Rapid cloning of R genes

R genes N B S L R R

PCR target plantR gene Analogs

Clone PCRfragments

Gene silencing/Transpososon mutants

Loss of response to Avr/elicitor

Screen genomic library for RGAsUse positive clones in transformation

Gain of response to Avr/elicitor

Genetic engineering for resistanceNatural resistance in the gene for gene systemThe Avr9-Cf9 pair determines resistance in the interaction

between Cladosporium fulvum and tomato

Genetic engineering for resistanceEngineered resistance: the two component systemBoth Avr9 and Cf9 are introduced in a crop plant under the

control of a pathogen inducible promoter. HR occurs uponinfection by pathogen

Summary R genes are the central players in plant innate immunity

or the ability of plants to defend against pathogeninfection

The gene for gene model and its modern illustration theguard model define R gene mediated resistance

R gene mediated resistance typically functions throughactivation of the Hypersensitive Response, a ubiquitousdefense response

R genes are structurally diverse and complex

R genes can be exploited to engineer resistant plants

bacterial and fungal host specificity plant pathology 602 ...plant pathology 602 plant-microbe...

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